netfilter: conntrack: switch connlabels to atomic_t

The spinlock is back from the day when connabels did not have
a fixed size and reallocation had to be supported.

Remove it. This change also allows to call the helpers from
softirq or timers without deadlocks.

Also add WARN()s to catch refcounting imbalances.

Signed-off-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/net/netfilter/nf_conntrack_labels.h

···
 #ifdef CONFIG_NF_CONNTRACK_LABELS
 	struct net *net = nf_ct_net(ct);
 
-	if (net->ct.labels_used == 0)
+	if (atomic_read(&net->ct.labels_used) == 0)
 		return NULL;
 
 	return nf_ct_ext_add(ct, NF_CT_EXT_LABELS, GFP_ATOMIC);

include/net/netns/conntrack.h

···
 	struct nf_ct_event_notifier __rcu *nf_conntrack_event_cb;
 	struct nf_ip_net	nf_ct_proto;
 #if defined(CONFIG_NF_CONNTRACK_LABELS)
-	unsigned int		labels_used;
+	atomic_t		labels_used;
 #endif
 };
 #endif

net/netfilter/nf_conntrack_labels.c

···
 #include <net/netfilter/nf_conntrack_ecache.h>
 #include <net/netfilter/nf_conntrack_labels.h>
 
-static DEFINE_SPINLOCK(nf_connlabels_lock);
-
 static int replace_u32(u32 *address, u32 mask, u32 new)
 {
 	u32 old, tmp;
···
 
 int nf_connlabels_get(struct net *net, unsigned int bits)
 {
+	int v;
+
 	if (BIT_WORD(bits) >= NF_CT_LABELS_MAX_SIZE / sizeof(long))
 		return -ERANGE;
 
-	spin_lock(&nf_connlabels_lock);
-	net->ct.labels_used++;
-	spin_unlock(&nf_connlabels_lock);
-
 	BUILD_BUG_ON(NF_CT_LABELS_MAX_SIZE / sizeof(long) >= U8_MAX);
+
+	v = atomic_inc_return_relaxed(&net->ct.labels_used);
+	WARN_ON_ONCE(v <= 0);
 
 	return 0;
 }
···
 
 void nf_connlabels_put(struct net *net)
 {
-	spin_lock(&nf_connlabels_lock);
-	net->ct.labels_used--;
-	spin_unlock(&nf_connlabels_lock);
+	int v = atomic_dec_return_relaxed(&net->ct.labels_used);
+
+	WARN_ON_ONCE(v < 0);
 }
 EXPORT_SYMBOL_GPL(nf_connlabels_put);